1. Field of the Invention
The disclosure relates to an air-fuel ratio control apparatus for an internal combustion engine.
2. Discussion of the Background
Japanese Unexamined Patent Application Publication No. 2000-220489 discloses a control apparatus that determines a variation in air-fuel ratio of each cylinder using a single air-fuel ratio sensor provided in the pipes-assembled portion of an exhaust manifold of an internal combustion engine having a plurality of cylinders. This control apparatus acquires data to be used in the determination when a predetermined condition including the amount of a change in the intake air flow rate being smaller than a predetermined amount, the intake air flow rate lying within the range of predetermined upper and lower limits, and the amount of a change in engine speed being smaller than a predetermined amount is fulfilled.
Further, the control apparatus calculates the intensity (MPOW1) of a one-cycle frequency component (frequency component equivalent to a half of a frequency corresponding to the engine speed) which is calculated based on the acquired data, and determines that the air-fuel ratio for each cylinder is varying beyond an allowable limit, when the intensity of the frequency component is equal to or larger than a threshold value (THMP1).
The control apparatus according to the related art executes the determination which uses the amount of a change in an operational state parameter XOP (intake air flow rate, engine speed) of the engine by comparing the amount of a change DXOP for a constant time DT (e.g., 100 msec) with a predetermined amount DXOPTH. Even in case of an operational state where there is a significant change in a period of 300 msec or so, for example, data acquisition is permitted when the change DXOP is equal to or smaller than the predetermined amount DXOPTH (see FIG. 14A). Further, even when the operational state parameter XOP has varied significantly, data acquisition is permitted when the variation lies within the range of predetermined upper and lower limits XOPLMH and XOPLML (see FIG. 14B).